1. Field of the Invention
The present invention relates to a printer for forming an image on a recording medium such as a recording sheet based on image data, and also relates to a method for controlling the printer.
2. Description of the Related Art
Printers are classified according to the method of image creation, such as thermal transfer printers, ink jet printers, laser printers, and wire dot printers. Thermal transfer printers perform dot line printing on a recording sheet with an ink ribbon. This type of printer has a plurality of heaters arranged in the main scanning direction. The heaters are selectively heated while the ink ribbon and the recording sheet are conveyed in the sub-scanning direction. Recently, with advances in image input devices such as digital cameras, digital camcorders, and image scanners, thermal transfer printers have received a lot of attention. Thermal transfer printers are suitable for outputting electronic image data obtained with a digital still camera or a digital camcorder via a personal computer or a data storage medium.
Other printers, such as ink jet printers, have only two (binary) options, forming a dot or not forming a dot. Therefore, resolution and gradation are achieved typically by the error diffusion technique using small dots. On the other hand, the thermal transfer printer is capable of pixel-by-pixel shading by varying the amount of heat. Therefore, the thermal transfer printer achieves smoother and higher quality images in comparison with other printer types such as ink jet printers. In addition, due to improvements in thermal heads and materials used for the recording sheets, the thermal transfer printer has achieved image printing quality comparable to a silver salt photograph. In step with the advances in digital cameras, thermal transfer printers are now attracting interest, especially as printers for natural images.
A system capable of printing by using a data storage medium without connecting a thermal transfer printer and an imaging device, such as a digital camera and a digital camcorder, has been developed. In addition, a system capable of printing without using a personal computer but by connecting a thermal transfer printer and the imaging device has also been developed. These systems make it easy to print out image data from a digital camera or a digital camcorder. Therefore, increasing attention is being paid to thermal transfer printers.
FIG. 10 is a schematic view showing a printing mechanism of a conventional compact thermal transfer printer. Pressed between a platen roller 107 and a thermal head 109, a recording sheet P and an ink ribbon 110 come into contact with each other. Ink on the ink ribbon 110 is transferred onto the recording sheet P by heat generated by the thermal head 109. The recording sheet P is conveyed by a pair of rollers 105A and 105B disposed forward of the thermal head 109 in the conveying direction. Printing is thus performed.
In order to recreate desired colors, it is necessary to repeat printing in a plurality of colored inks, such as yellow, magenta, and cyan. Therefore, printing takes a long time in comparison with other printers. In addition, after printing in the first color is completed, in order to perform printing in the next color, it is necessary to release the recording sheet P from the pressure of the thermal head 109, to rotate the rollers 105A and 105B in reverse, and to return the recording sheet P to the starting position. This makes the printing time much longer.
To solve this problem, a printer is disclosed in Japanese Patent Laid-Open No. 2003-39760. In this printer, the printing time is reduced by reducing the time to return the recording sheet. The circumference of a platen roller is longer than a recording sheet. The recording sheet is conveyed along the circumference of the platen roller. The recording sheet is pressed against the platen roller by a pushing member. With rotation of the platen roller, the recording sheet is conveyed due to friction with the platen roller in the rotating direction of the platen roller.
When printing in the first color is completed, the leading edge of the recording sheet is short of the thermal head. Rotating the platen roller slightly further returns the recording sheet to the starting position. Printing in the next color is thus performed smoothly. Since this printer requires very little time to return the recording sheet in comparison with the conventional thermal transfer printer, the printing time is reduced.
However, this printer requires increasing the size of the platen roller according to the size of the recording sheet. Therefore, the size of the printer body is also increased. In addition, in the case where a wide variety of recording sheets are to be used and there is a great difference in length between the largest size and the smallest size, the platen roller tailored for the largest size of recording sheet cannot quickly return the smaller sizes of recording sheets.